This invention relates to the art of planographic printing and particularly to that of offset lithography. More particularly, the invention pertains to a dampening mechanism for use in offset printing presses, featuring provisions for adjustably varying the rate of supply of moistening water. The term "water" as used in this specification and in the claims appended thereto should be taken to mean not just water but also dampening solutions of any known or suitable compositions consisting mainly of water.
Planography, as is well known, is an art of nonrelief printing such that both the image and nonimage areas of the printing plate are on the same printing surface. The two areas do, however, differ from each other in that the image area is oleophilic (i.e. attracts the greasy ink and repels water) whereas the nonimage area is hydrophilic (i.e. water-attractive and ink-repellent). As the face of the printing plate, clamped around the plate cylinder, is moistened, the nonimage areas of the plate accepts the moisture. The plate is inked by passing under the inking mechanism here the image areas accept the ink. In offset printing, the inked image is transferred from the plate to a blanket on a blanket cylinder and thence retransferred or offset to a web of paper or the like.
Whereas a variety of apparatuses have been suggested and used for dampening the printing plates in offset printing presses, those employing sprayers are winning ever-increasing acceptance in the industry. A sprayer sprays water either on to the printing plate on the plate cylinder or on to a water roller that makes rolling contact, either directly or indirectly, with the plate cylinder.
Japanese Unexamined Utility Model Application No. 63-154225 represents one such conventional approach, teaching a baffle plate cantilevered to a rotary shaft above water sprayers. The baffle plate is angled up and down to regulate the water spray and hence to control the rate at which the water is supplied.
A drawback of this prior art device is that the baffle plate is directly under the pressure of the water being sprayed. The rotary shaft to which the baffle plate is cantilevered is geared to its drive means. Therefore, even under the infinitesimal fluctuations of the spray pressure, the pivoted baffle plate has been susceptible to oscillations within the limitations of the backlash of the drive gearing. Such oscillations of the baffle plate have rendered the control of the moistening rate inaccurate and unreliable. After an extended period of use, moreover, the baffle plate oscillations have often resulted in the malfunctioning of the device through abrasion of the rotary shaft and the parts supporting the same.
Japanese Unexamined Patent Application No. 5-16334 suggests the provision of a solenoid-actuated flow control valve on a conduit leading to a water sprayer. The solenoid valve is automatically opened and closed at intervals depending upon the operating speed of the printing press, so that water is sprayed at intervals either on the plate itself or on a roller in contact therewith.
This known spray system possesses the drawback that the supply rate of water is varied only in discrete steps rather than infinitely. Additionally, it is incapable of varying the supply rate by quantities less than those resulting from the minimum possible intervals at which the solenoid valve is opened and closed.
Japanese Utility Model Publication No. 39-28072 teaches the spraying of water in mixture with air under pressure. The spray rate is controlled by inching the sprayer back and forth in the direction of the spray and by controlling the valves on conduits that convey the water and the compressed air toward the sprayer. It is possible in this manner to adjustably vary not only the rate of spray but the size of the spray drops.
However, when the water pressure is raised for a higher rate of supply, the air pressure may be likely overcome by the water pressure, failing to atomize the water into sufficiently fine particles. With the water thus sprayed in coarse drops, the plate or the water roller often is not uniformly moistened, resulting in uneven printing or smearing of the paper. It must also be pointed out that the adjustment of the moistening rate through control of the water and air pressures is very difficult and demands utmost skill on the part of the press operators.